It is known practice for the current in a current distribution system to be distributed to individual installation paths (loads) by way of a switch arrangement. The switches, particularly circuit breakers for the low voltage domain, which are usually arranged in groups or distribution levels, are each designed for a rated current and interrupt the current flowing through the switch in the event of a fault, that is to say when there is a short circuit, for example.
To this end, they operate a latching solenoid with a displaceable push rod (maglatch), for example, that acts on a prestressed breaker shaft that opens the switch. Only the installation path that is affected by the fault or that is closest to the fault is turned off in each case. This behavior is called selective turning off.
Each switch contains a transformer and a trip unit. The transformer has a wire coil (Rogowski coil), for example, and senses the current that flows through the switch, and the trip unit checks whether a prescribed current condition is met.
For the purpose of selective turning off (also known as zone selective interlocking ZSI), the switches arranged in groups communicate with one another via appropriate links (communication links) that each connect the switches in a downstream group to the switches in the directly upstream group. The links may be in the form of two-wire lines or in the form of bus links and the like.
Thus, a—as seen from the supply—downstream switch (in a downstream group) whose current condition is met communicates this to the—as seen from the supply—directly upstream switch (in the directly upstream group) by means of an appropriate signal (blocking signal, delay signal). As a result, the upstream switch, which likewise detects the short circuit on account of the continuous flow of energy, does not trip itself for the time being, but rather waits a particular time (a prescribed delay time) to determine whether the downstream switch trips. If the downstream switch has not tripped after the delay time has elapsed, the upstream switch interrupts the flow of current itself.
The communication path is in the direction of the supply in each case, that is to say contrary to the direction of energy flow, from the downstream switch to the directly upstream switch (in each case from a downstream group to the directly upstream group).
If, when the current condition is met, a downstream switch establishes that it has a problem that prevents tripping, e.g. a fault in the form of a blocked or faulty maglatch or a fractured wire in a Rogowski coil, it can distribute this information, possibly by way of communication in the system, but this does not rectify the fault directly. Particularly in the case of small overcurrents that are not detected as a fault current in the upstream switch on account of their low level, there is the risk of the faulty switch severely overheating (and of there being a fire in the switch).